FeLi[PO4]: Dissection of a Crystal Structure - Inorganic 3D Structures - page 78

Our discussion will be focused on the similarity of the cation arrays with the
Ni 2 In-type structure (Fig. 9a ) and more precisely with the NiAs-type skeleton
which is implicit in it. Special emphasis will be given to the characteristics defining
each phase.
The differences between the cation arrays of both phases of FeLiPO 4 are simple.
As discussed above, triphylite FeLi[PO 4 ] was described as a Fe-stuffed [Li C -Cl]

array of the NiAs type in which the P atoms ( -Cl) form a hcp array with all the

octahedral voids filled by the Li atoms (Fig. 5a ). Remember also that the pseudo-
structure of

C

-Li[ClO 4 ] in triphylite showed a great distortion with respect to that
of real Li[ClO 4 ][ 21 , 22 ]. On the contrary, in

C

-FeLi[PO 4 ], the octahedra of the hcp
array of P atoms are now filled by the Fe atoms, forming so a Li-stuffed [FeP]
subarray (Fig. 9b ) .

b

3.1 Relationships of b -FeLi[PO 4 ] with Fe[SO 4 ] and Li[ClO 4 ]

If the EZKC [ 4 ] is applied to -FeLiPO 4 , the structure can be rationalized by

assuming the one-electron transfer from the Li atom to the P atom which is
converted into a

b

C -S. In this way, b -FeLiPO 4 should be formulated either as

Li + [FeP] or as (

C -He)[Fe C -S]. The result is that the cation array of the b -phase

would contain, in fact, a -FeS substructure of the NiAs type [ 25 ]. In this

arrangement, the Li atoms are located in trigonal bipyramids, as the Fe atoms did
in triphylite. Consequently,

C

b -FeLi[PO 4 ] can then be formulated as a Li-stuffed

C -Fe[SO 4 ] structure. This view of the structure is represented in Fig. 9b .

By examining the structure of Fe[SO 4 ][ 33 ], also mcm, one sees that, once

more, the structure of the pseudo-formula (Fig. 9b ) coincides with that of real
Fe[SO 4 ], which is represented in Fig. 9c . Moreover, if in both compounds the
O atoms are neglected, the FeS subarrays of both compounds are identical to the
high temperature phase of real FeS (also

C

mcm) [ 34 ] . Again, the structure, as well
as the space group, is maintained in FeS, Fe[SO 4 ] and

C

C

-Fe[SO 4 ]! The three

structures are shown in Fig. 9c, b and a , respectively.
The lack of similarity between the two phases, triphylite and

-FeLi[PO 4 ],
becomes evident by comparing Fig. 9b with Fig. 5a (the colour code is the same
in both figures). Although both drawings represent Li-stuffed NiAs-type struc-
tures, the important issue is that in

b

-FeLi[PO 4 ](Fig. 9b )the[FePO 4 ]subarray
has formed a NiAs-type arrangement which is much more regular than that
formed by the [LiPO 4 ] subarray in triphylite (compare with Fig. 5a ) . Thus,
although both structures are formed by an hcp array of PO 4 groups, the
chemical nature of the stuffing atoms (Fe and Li, respectively) seem to play a
decisive role in the formation of a “true” NiAs-type structure; that is, the
structure is close to the hexagonal regularity in

b

-FeLi[PO 4 ](Fig. 9b ) but is
highly distorted in triphylite (Fig. 5a ). The reasons for these differences will be
discussed below.

b